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ngrep − network grep


ngrep <-hNXViwqpevxlDtTRM> <-IO pcap_dump > < -n num > < -d dev > < -A num > < -s snaplen > < -S limitlen > < -W normal|byline|single|none > < -c cols > < -P char > < -F file > < match expression > < bpf filter >


ngrep strives to provide most of GNU grep’s common features, applying them to the network layer. ngrep is a pcap-aware tool that will allow you to specify extended regular expressions to match against data payloads of packets. It currently recognizes TCP, UDP and ICMP across Ethernet, PPP, SLIP, FDDI and null interfaces, and understands bpf filter logic in the same fashion as more common packet sniffing tools, such as tcpdump(8) and snoop(1).



Display help/usage information.


Show sub-protocol number along with single-character identifier (useful when observing raw or unknown protocols).


Treat the match expression as a hexadecimal string. See the explanation of match expression below.


Display version information.


Ignore case for the regex expression.


Match the regex expression as a word.


Be quiet; don’t output any information other than packet headers and their payloads (if relevant).


Don’t put the interface into promiscuous mode.


Show empty packets. Normally empty packets are discarded because they have no payload to search. If specified, empty packets will be shown, regardless of the specified regex expression.


Invert the match; only display packets that don’t match.


Dump packet contents as hexadecimal as well as ASCII.


Make stdout line buffered.


When reading pcap_dump files, replay them at their recorded time intervals (mimic realtime).


Print a timestamp in the form of YYYY/MM/DD HH:MM:SS.UUUUUU everytime a packet is matched.


Print a timestamp in the form of +S.UUUUUU, indicating the delta between packet matches. Specify a second time to indicate the delta since the first packet match.


Do not try to drop privileges to the DROPPRIVS_USER.

ngrep makes no effort to validate input from live or offline sources as it is focused more on performance and handling large amounts of data than protocol correctness, which is most often a fair assumption to make. However, sometimes it matters and thus as a rule ngrep will try to be defensive and drop any root privileges it might have.

There exist scenarios where this behaviour can become an obstacle, so this option is provided to end-users who want to disable this feature, but must do so with an understanding of the risks. Packets can be randomly malformed or even specifically designed to overflow sniffers and take control of them, and revoking root privileges is currently the only risk mitigation ngrep employs against such an attack. Use this option and turn it off at your own risk.

-c cols

Explicitly set the console width to ‘‘cols’’. Note that this is the console width, and not the full width of what ngrep prints out as payloads; depending on the output mode ngrep may print less than ‘‘cols’’ bytes per line (indentation).

-F file

Read in the bpf filter from the specified filename. This is a compatibility option for users familiar with tcpdump. Please note that specifying ‘‘-F’’ will override any bpf filter specified on the command-line.

-P char

Specify an alternate character to signify non-printable characters when displayed. The default is ‘‘.’’.

-K num

Kill matching TCP connections (like tcpkill). The numeric argument controls how many RST segments are sent.

-W normal|byline|single|none

Specify an alternate manner for displaying packets, when not in hexadecimal mode. The ‘‘byline’’ mode honors embedded linefeeds, wrapping text only when a linefeed is encountered (useful for observing HTTP transactions, for instance). The ‘‘none’’ mode doesn’t wrap under any circumstance (entire payload is displayed on one line). The ‘‘single’’ mode is conceptually the same as ‘‘none’’, except that everything including IP and source/destination header information is all on one line. ‘‘normal’’ is the default mode and is only included for completeness. This option is incompatible with ‘‘-x’’.

-s snaplen

Set the bpf caplen to snaplen (default 65536).

-S limitlen

Set the upper limit on the size of packets that ngrep will look at. Useful for looking at only the first N bytes of packets without changing the BPF snaplen.

-I pcap_dump

Input file pcap_dump into ngrep. Works with any pcap-compatible dump file format. This option is useful for searching for a wide range of different patterns over the same packet stream.

-O pcap_dump

Output matched packets to a pcap-compatible dump file. This feature does not interfere with normal output to stdout.

-n num

Match only num packets total, then exit.

-d dev

By default ngrep will select a default interface to listen on. Use this option to force ngrep to listen on interface dev.

-A num

Dump num packets of trailing context after matching a packet.

match expression

A match expression is either an extended regular expression, or if the -X option is specified, a string signifying a hexadecimal value. An extended regular expression follows the rules as implemented by the GNU regex library. Hexadecimal expressions can optionally be preceded by ‘0x’. E.g., ‘DEADBEEF’, ‘0xDEADBEEF’.

bpf filter

Selects a filter that specifies what packets will be dumped. If no bpf filter is given, all IP packets seen on the selected interface will be dumped. Otherwise, only packets for which bpf filter is ‘true’ will be dumped.

The bpf filter consists of one or more primitives. Primitives usually consist of an id (name or number) preceded by one or more qualifiers. There are three different kinds of qualifier:


qualifiers say what kind of thing the id name or number refers to. Possible types are host, net and port. E.g., ‘host blort’, ‘net 1.2.3’, ‘port 80’. If there is no type qualifier, host is assumed.


qualifiers specify a particular transfer direction to and/or from id. Possible directions are src, dst, src or dst and src and dst. E.g., ‘src foo’, ‘dst net 1.2.3’, ‘src or dst port ftp-data’. If there is no dir qualifier, src or dst is assumed. For ‘null’ link layers (i.e. point to point protocols such as slip) the inbound and outbound qualifiers can be used to specify a desired direction.


qualifiers are restricted to ip-only protocols. Possible protos are: tcp , udp and icmp. e.g., ‘udp src foo’ or ‘tcp port 21’. If there is no proto qualifier, all protocols consistent with the type are assumed. E.g., ‘src foo’ means ‘ip and ((tcp or udp) src foo)’, ‘net bar’ means ‘ip and (net bar)’, and ‘port 53’ means ‘ip and ((tcp or udp) port 53)’.

In addition to the above, there are some special ‘primitive’ keywords that don’t follow the pattern: gateway, broadcast, less, greater and arithmetic expressions. All of these are described below.

More complex filter expressions are built up by using the words and, or and not to combine primitives. E.g., ‘host blort and not port ftp and not port ftp-data’. To save typing, identical qualifier lists can be omitted. E.g., ‘tcp dst port ftp or ftp-data or domain’ is exactly the same as ‘tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain’.

Allowable primitives are:
dst host

True if the IP destination field of the packet is host, which may be either an address or a name.

src host host

True if the IP source field of the packet is host.

host host

True if either the IP source or destination of the packet is host. Any of the above host expressions can be prepended with the keywords, ip, arp, or rarp as in:

ip host host

which is equivalent to:

ether dst ehost

True if the ethernet destination address is ehost. Ehost may be either a name from /etc/ethers or a number (see ethers(3N) for numeric format).

ether src ehost

True if the ethernet source address is ehost.

ether host ehost

True if either the ethernet source or destination address is ehost.

gateway host

True if the packet used host as a gateway. I.e., the ethernet source or destination address was host but neither the IP source nor the IP destination was host. Host must be a name and must be found in both /etc/hosts and /etc/ethers. (An equivalent expression is

ether host ehost and not host host

which can be used with either names or numbers for host / ehost.)

dst net net

True if the IP destination address of the packet has a network number of net. Net may be either a name from /etc/networks or a network number (see networks(4) for details).

src net net

True if the IP source address of the packet has a network number of net.

net net

True if either the IP source or destination address of the packet has a network number of net.

net net mask mask

True if the IP address matches net with the specific netmask. May be qualified with src or dst.

net net/len

True if the IP address matches net a netmask len bits wide. May be qualified with src or dst.

dst port port

True if the packet is ip/tcp or ip/udp and has a destination port value of port. The port can be a number or a name used in /etc/services (see tcp(4P) and udp(4P)). If a name is used, both the port number and protocol are checked. If a number or ambiguous name is used, only the port number is checked (e.g., dst port 513 will print both tcp/login traffic and udp/who traffic, and port domain will print both tcp/domain and udp/domain traffic).

src port port

True if the packet has a source port value of port.

port port

True if either the source or destination port of the packet is port. Any of the above port expressions can be prepended with the keywords, tcp or udp, as in:

tcp src port port

which matches only tcp packets whose source port is port.

less length

True if the packet has a length less than or equal to length. This is equivalent to:

len <= length.

greater length

True if the packet has a length greater than or equal to length. This is equivalent to:

len >= length.

ip proto protocol

True if the packet is an ip packet (see ip(4P)) of protocol type protocol. Protocol can be a number or one of the names tcp, udp or icmp. Note that the identifiers tcp and udp are also keywords and must be escaped via backslash (\), which is \\ in the C-shell.

ip broadcast

True if the packet is an IP broadcast packet. It checks for both the all-zeroes and all-ones broadcast conventions, and looks up the local subnet mask.

ip multicast

True if the packet is an IP multicast packet.


Abbreviation for:

ether proto ip

tcp, udp, icmp

Abbreviations for:

ip proto p

where p is one of the above protocols.

expr relop expr

True if the relation holds, where relop is one of >, <, >=, <=, =, !=, and expr is an arithmetic expression composed of integer constants (expressed in standard C syntax), the normal binary operators [+, -, *, /, &, |], a length operator, and special packet data accessors. To access data inside the packet, use the following syntax:

proto [ expr : size ]

Proto is one of ip, tcp, udp or icmp, and indicates the protocol layer for the index operation. The byte offset, relative to the indicated protocol layer, is given by expr. Size is optional and indicates the number of bytes in the field of interest; it can be either one, two, or four, and defaults to one. The length operator, indicated by the keyword len, gives the length of the packet.

For example, ‘ether[0] & 1 != 0’ catches all multicast traffic. The expression ‘ip[0] & 0xf != 5’ catches all IP packets with options. The expression ‘ip[6:2] & 0x1fff = 0’ catches only unfragmented datagrams and frag zero of fragmented datagrams. This check is implicitly applied to the tcp and udp index operations. For instance, tcp[0] always means the first byte of the TCP header, and never means the first byte of an intervening fragment.

Primitives may be combined using:

A parenthesized group of primitives and operators (parentheses are special to the Shell and must be escaped).

Negation (‘!’ or ‘not’).

Concatenation (‘&&’ or ‘and’).

Alternation (‘||’ or ‘or’).

Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. Note that explicit and tokens, not juxtaposition, are now required for concatenation.

If an identifier is given without a keyword, the most recent keyword is assumed. For example,

not host vs and ace

is short for

not host vs and host ace

which should not be confused with

not ( host vs or ace )

Expression arguments can be passed to ngrep as either a single argument or as multiple arguments, whichever is more convenient. Generally, if the expression contains Shell metacharacters, it is easier to pass it as a single, quoted argument. Multiple arguments are concatenated with spaces before being parsed.


Errors from ngrep, libpcap, and the GNU regex library are all output to stderr.


The ngrep utility exits with one of the following values:

0 One or more frames were matched.
1 No frames were matched.
2 An error occurred.
3+ Hell is freezing over, run!


Written by Jordan Ritter <jpr5@darkridge.com>.


Please report bugs to the ngrep’s GitHub Issue Tracker, located at


Non-bug, non-feature-request general feedback should be sent to the author directly by email.